504: CHEMICAL ENGINEERING
These figures show the point brought out with unmistakable clearness. It will be noted that the fire shrinkage of the German clay is far less than that of the American Kond clays.
Assuming that the mean pot arch temperature is 1,050°C., which is above the point usually reached, and that the mean furnace temperature is 1,290°C., the Grossal-merode clay will contract about 7 per cent of the dry volume; the St. Louis clay, 14.55; the Tennessee ball clay, 24.85; the Kentucky ball clay, 30.26; and the Ohio material, 18.94 per cent. Thus, allowing for a grog content of 50 per cent, the contraction in the furnace of a pot having a solid content of 10 cu. ft. would vary from 0.75 to 1.5 cu. ft. of displacement, a condition which taxes the strength of the body to the limit. The figures given show also one reason why the Missouri clays have been preferred to pot making, their fire shrinkage being the lowest of the American clays cited. It is evident, therefore, that the use of a siliceous clay like that from Arkansas with a fire shrinkage of only 1.6 per cent at 1,290°C. is greatly to be desired. Its exclusive use, however, is not desirable, due to its too open structure, and its combination with a clay of the ball clay type should prove eminently satisfactory.
The large furnace shrinkage of American bond clays may be overcome by admixture with a siliceous clay or sand, by increasing the content of grog or by sizing, and by the employment of higher prefiring furnaces.
The use of siliceous clays probably offers the easiest remedy, since materials of this type are plentiful and are to be found in large quantities in New Jersey, as well as associated with many of the fireclay deposits of Missouri, Kentucky, Tennessee, Ohio, Pennsylvania, and other States. It is not desirable to reduce the shrinkage of the clay or the pot body to its lowest possible value; but the fire contraction of the blended clay mixture (without grog) should be about 12 to 15 per cent, in terms of the dry volume, at 1,300°C. It is obvious that exact limits cannot be given to cover all cases.
For the purpose of bringing about a high degree of density and imperviousness the addition of feldspar to the body has been introduced by Scholes, a practice which has met with good success in glass-melting pots. For such uses as zinc retorts the Missouri bond clays with grog from the same source, or sometimes calcined flint clay, are largely used. In all cases the bond clay used must be sufficiently refractory and must not have a softening point below that of cone 31, excepting in the case of siliceous clays which may go as low as cone 28. In addition they must have the required degree of plasticity, and strength in the dry and the fired state.
Special Refractories of the Porcelain Type.—For certain purposes, as the melting of optical glass, of fine enamels, color fluxes, etc., where the presence of iron is undesirable, white bodies may be prepared through the use of grog prepared from a mixture of 80 per cent kaolin, 10 per cent ground quartz and 10 per cent of feldspar. This grog may be replaced by the use of waste pottery bisque (unglazed porous table ware). Mixtures of this type are as follows:
Kaolin grog.............................. 40
White ware bisque........................ .. 48.0
Raw kaolin............................... 22 23.5
Plastic bond clay.......................... 23 22.0
Feldspar................................. 7 6.5
Ground quartz............................ 8
These and similar mixtures should be brought close to vitrification in order to develop their resistance to corrosion to the maximum where this quality is desired.